The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Oil and gas hydrocarbons are naturally occurring in some subterranean formations. A subterranean formation containing oil or gas is sometimes referred to as a reservoir. A reservoir may be located under land or off shore. Reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs). In order to produce oil or gas, a wellbore is drilled into a reservoir or adjacent to a reservoir.
A well can include, without limitation, an oil, gas, or water production well, or an injection well. As used herein, a “well” includes at least one wellbore. A wellbore can include vertical, inclined, and horizontal portions, and it can be straight, curved, or branched. As used herein, the term “wellbore” includes any cased, and any uncased, open-hole portion of the wellbore. A near-wellbore region is the subterranean material and rock of the subterranean formation surrounding the wellbore. As used herein, a “well” also includes the near-wellbore region. The near-wellbore region is generally considered to be the region within about 100 feet of the wellbore. As used herein, “into a well” means and includes into any portion of the well, including into the wellbore or into the near-wellbore region via the wellbore.
A portion of a wellbore may be an open hole or cased hole. In an open-hole wellbore portion, a tubing string may be placed into the wellbore. The tubing string allows fluids to be introduced into or flowed from a remote portion of the wellbore. In a cased-hole wellbore portion, a casing is placed into the wellbore, which can also contain a tubing string. A wellbore can contain an annulus. Examples of an annulus include, but are not limited to: the space between the wellbore and the outside of a tubing string in an open-hole wellbore; the space between the wellbore and the outside of a casing in a cased-hole wellbore; and the space between the inside of a casing and the outside of a tubing string in a cased-hole wellbore.
During well completion, it is common to introduce a cement composition into an annulus in a wellbore. For example, in a cased-hole wellbore, a cement composition can be placed into and allowed to set in an annulus between the wellbore and the casing in order to stabilize and secure the casing in the wellbore. By cementing the casing in the wellbore, fluids are prevented from flowing into the annulus. Consequently, oil or gas can be produced in a controlled manner by directing the flow of oil or gas through the casing and into the wellhead. Cement compositions can also be used in primary or secondary cementing operations, well-plugging, squeeze cementing, or gravel packing operations.
It is common to include a filler in a cement composition. The filler can help reduce the overall cost of the cement composition. One type of filler that is commonly included in a cement composition is a pozzolan. As used herein, a “pozzolan” is a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form and in the presence of water, chemically react with a source of calcium at a temperature of 71° F. (22° C.) to form compounds possessing cementitious properties.
As used herein, the phrase “cementitious properties” means the ability to bind materials together and set. It is to be understood that the term “pozzolan” does not necessarily indicate the exact chemical make-up of the material, but rather refers to its capability of reacting with a source of calcium and water to form compounds possessing cementitious properties. When a pozzolan is mixed with water, the silicate phases of the pozzolan can undergo a hydration reaction and form hydration products of calcium silicate hydrate (often abbreviated as C-SH) and also possibly calcium aluminate hydrate. A pozzolan in general is less expensive than cement and can generally be included in a cement composition up to about 40% by weight of the cement. Therefore, a pozzolan can not only decrease the overall cost of the cement composition, but also will not adversely affect the desirable properties of the cement composition (e.g., the compressive strength or setting time).
The degree to which a material functions as a pozzolan can be determined by the pozzolanic activity of the material. The pozzolanic activity of a pozzolan is the reaction rate between the pozzolan and a source of calcium (e.g., Ca2+, calcium oxides “CaO”, or calcium hydroxides “Ca(OH)2”) in the presence of water. The pozzolanic activity can be measured by determining the amount of calcium the pozzolan consumes over time or by determining the compressive strength of a pozzolan composition containing the pozzolan and water or a cement composition containing cement, the pozzolan, a source of calcium, and water.
Strength retrogression is a decline in the compressive strength of a cement composition over time, especially at elevated temperatures. The decline is more pronounced at temperatures above 230° F. (110° C.). Therefore, it is common to include a strength retrogression inhibitor in a cement composition. Strength retrogression inhibitors can function to inhibit or prevent a decline of the compressive strength of a cement composition over time. However, pozzolans are generally not considered to be strength retrogression inhibitors. This means that in order to reduce the cost of a cement composition while still maintaining the desirable properties of the composition, both a pozzolan and a strength retrogression inhibitor must be included in the composition for use in higher-temperature wells. The addition of two separate additives may not reduce the cost as much as may be desirable and requires more time by having to incorporate both additives into the cement composition.
Typically, fly ash, silica fume, metakaolin and pumice have been used as pozzolans. However, consistency problems can occur because samples can have many origin points. Therefore, a need exists for a single origin pozzolan that can also act as a strength retrogression inhibitor.